200 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 10, 



which " differ remarkably in size and form," being caulescent, scopi- 

 form, ramose, dendritic, claviform, or sheaf-like, and resembling in 

 many respects certain cryptocrystalline aggregations known to mi- 

 neralogists. They occur in immediate contact with the " chamber- 

 casts," rising directly out of them, occasionally resting against the 

 asbestiform layer, likewise seemingly in no way connected with 

 anything, being in appearance isolately imbedded in the calcite of 

 the septa*. In general their physical characters have a resemblance 

 to those of the "white amorphous masses," excepting that they are 

 somewhat more compact, and have a lustre and colour, especially in 

 their centre, approaching to the translucency and green tint of the 

 *' chamber-casts." 



Comparing the " amorphous masses " with the " definite shapes,'* 

 there is certainly considerable dissimilarity between them ; never- 

 theless the more examples we examined, the more we felt ourselves 

 constrained to agree with Dr. Carpenter that both are modifications 

 of one typef. But as regards their origin, we are under the necessity 

 of entirely discarding the opinion which ascribes it to organic 

 structures. 



Previously, however, to going into this question, we deem it right 

 to show that similar bodies, more or less agreeing with the *' amor- 

 phous masses " and " definite shapes," are not uncommon as imbedded 

 crystalline aggregations. The subject hitherto has received but httle 

 attention, all that is known of it being confined to a few papers 

 which seem to have attracted scarcely any notice. 



As far back as 1813 and 1815, Dr. MaccuUoch brought under the 

 notice of the Geological Society some singular vermicular " forms 

 in which chlorite is disposed," occurring in chalcedony and quartz $ ; 

 and in 1849, Prancis Alger briefly described some similar " imitative 

 shapes " of mica enclosed in crystals of quartz from Yermont, which, 

 had they occurred in " eozoonal rock," would have been taken for 

 casts of tortuous canals §. Native metals, notably copper and sUver, 

 likewise metallic oxides, are well known to occur as imbedded arbo- 

 rescences||. The Museum of Trinity College, Dublin, contains a 

 beautiful arborescent example of native silver, imbedded in calcite, 

 strikingly resembling the dendritic ramifications in Grenville Ophite ; 

 and there is now before us a singularly apposite specimen, of a less 

 complex character however, for which we are indebted to Dr. Prazer 

 of Dublin, consisting of slender simple as well as branching stems 

 of pyrites, imbedded in the same kind of matrix. But it is from 

 decalcification that the most important results may be expected to 

 be obtained in our favour. Many of the examples of the so-called 

 " siliceous skeleton," got by M. Alphonse Gages from various rock- 



* In polished specimens of Grenville Ophite, both the " amorphous masses " 

 and the " definite shapes " may with a good pocket-lens often be detected lying 

 imbedded in the calcite of the septa, from which they are well distinguished by 

 their milk-white appearance. 



t See ' Intellectual Observer,' vol. vii. pp. 293, 294. 



I Trans. Geol. Soc. 1st ser. vol. ii. pi. 36. figs. 2 & 3 ; also vol. iv. pi. 27. fig. 1. 

 § American Journal of Science, 2nd ser. vol. x. pp. 14& 15. 



II Most specimens of the kind seen in cabinets are partially or wholly decalcified. 



